1. Field of the Invention
The present invention relates to a direct thermal printer which records images on a thermosensitive recording medium, and more particularly to a thermal printer wherein the thermosensitive recording medium is prevented from contacting light sources which are used for optical fixing of the thermosensitive recording medium.
2. Related Art
A direct thermal printer uses a thermosensitive recording medium having a thermosensitive layer formed on a supporting material. The thermosensitive layer is color-developed when heated directly by a thermal head. Because the thermosensitive layer still has a capacity of color-developing after the thermal printing of an image on the thermosensitive recording medium, it is possible that unnecessary parts of the thermosensitive recording medium are developed if a certain amount of heat energy is accidentally applied thereto. Therefore, the thermosensitive recording medium is exposed to electromagnetic rays, such as ultraviolet rays, after the thermal printing, so as to optically fix or neutralize the color-developability.
Also a thermosensitive color recording medium having three thermosensitive color recording layers of magenta, cyan and yellow is known, for example, in U.S. Pat. Nos. 4,734,704 and 4,833,488 (corresponding to Japanese Laid-open Patent Application No. 61-213169). The thermosensitive color recording layers are formed on one another on a supporting material, and a full-color image is recorded by sequentially developing the coloring layers in magenta, cyan and yellow.
Specifically, if the yellow recording layer is disposed on the top, yellow pixels of the full-color image are first recorded by developing this layer. Thereafter, the second layer from the top, for example, the magenta recording layer is developed to record magenta pixels of the full-color image. In order to prevent the yellow recording layer from being developed during the magenta pixel recording, optical fixing of the yellow recording layer is performed by exposing the thermosensitive color recording medium to electromagnetic rays, for example, near-ultraviolet rays of 420 nm. The magenta recording layer is also optically fixed after the magenta pixel recording, for example, by projecting ultraviolet rays of 65 nm onto the color recording medium.
The ultraviolet lamp used for the above-described optical fixing is disposed as close to the thermosensitive recording medium as possible, so that the effective amount of light projected from the ultraviolet lamp onto the thermosensitive recording medium is increased and the efficiency of the optical fixing is improved. For example, the distance between the ultraviolet lamp and the thermosensitive recording medium, which is wound on the platen drum, is conventionally set at about 5 mm.
Because the thermosensitive recording medium is secured at its leading end to the platen drum, but the trailing end is not secured, it is possible for the thermosensitive recording medium to be removed off of the platen drum from the trailing end and to contact the ultraviolet lamp. The part of the recording medium which contacts the ultraviolet lamp may be developed by heat energy radiated from the ultraviolet lamp.
In view of the foregoing, a primary object of the present invention is to provide a direct thermal printer wherein the thermosensitive recording medium is prevented from being contacted with a lamp of an optical fixing device.
Another object of the present invention is to provide a direct thermal printer wherein the distance between the thermosensitive recording medium and an optical fixing lamp is maintained at a constant distance.